How to retrieve the objects of a group
Each group in blender has an 'objects' property which stores all members of that group as a list.

In the context of your script this would loop through each group in the blend file and print the name of each object in that group:

import bpy

for group in bpy.data.groups:
    for object in group.objects:
        print(object.name)


Although, if you simply wanted to access the list, no looping is necessary, you can simply use:

bpy.data.groups['GroupName'].objects


You can then access objects with:
 ...objects[0].name